In windows for installation in an inclined roof and comprising a main frame and a top-hinged secondary frame element which in itself may be provided with one or more glass panes or in which a pane supporting tilting sash may be mounted it is known to balance at least part of the weight of the movable components by means of a lever inserted between a pair of associated side members of the main frame and the secondary frame element, respectively, and having one end pivotally connected with one of the side members while its other end is hinged to a sliding shoe displaceable along the other side member, said shoe being urged by a tension spring in a direction so as to exert an outwardly directed pressure through the lever on the secondary frame element. The purpose of this arrangement is to facilitate opening the window, and the dimensions may be chosen so that the spring can retain the top-hinged secondary frame in equilibrium in a desired opening position.
A deficiency of the prior art balancing mechanisms of said type constitutes in that the tension spring in and adjacent the closing position of the window only produces a very light lifting moment or possibly no lifting effect at all on the secondary frame element because the centre line of the tension spring passes through or is located very close to the hinge axis of the secondar frame element and cannot either be spaced far from the pivot point at the other end of the lever. This entails that the window at initial opening requires a considerable force action and that the secondary frame element when being reclosed strikes heavily against the main frame if efficient restraining provisions are not made. In other words, the curve of the balancing moment has an inappropriate course, and only in a single position the mechanism fully balances the secondary frame element.
A more suitable course of the lifting moment curve is obtained by a design in which a tension spring extending along a side member of a main frame or a sash has its lower end connected with a sash lifting lever while its upper end is secured to another lever, one end of which is likewise pivoted to the sash or the main frame. From its pivot point this lever extends upwardly towards the upper edge of the window and it slidingly engages the main frame or sash member and is held thereagainst by the spring due to the fact that the centre line of the spring even in this situation is spaced outwardly from the pivot points of said two levers on the sash or main frame.
By choosing appropriate dimensions for this prior art design the entire moment curve may follow at least approximately an ideal course but, in return, the structure is much more complicated than is the case with the first mentioned balancing or lifting mechanism.